Numerous motor applications require enclosure of the motor windings and operation in an environment in which significant concentrations of heat arise. In many of these applications deleterious particulates as dust are present in the environment from which it would be most beneficial to shield the motor windings. Typical of such applications are the use of ceiling mounted motors to operate ceiling mounted room fans and combined fan and lighting fixtures.
When a motor is forced to operate in a high temperature and "dirty" environment, among other things the motor may consume more power, and its working lifetime reduced. Additionally, the insulation on the winding conductors can be damaged or destroyed, possibly precipitating a fire. Ceiling mounted room fans, in particular, begin circulating warmer air, offsetting and defeating gains made in cooling the room by moving the otherwise stagnant air therein.
One solution to elimiate particles from collecting on the motor windings is to completely seal the same inside a housing. But since enclosing the windings of a motor within a housing significantly increases the motor operating temperature resulting in the above mentioned difficulties, historically motors have incorporated somewhat randomly placed holes in the motor housing through which environmental air may pass. However, these holes still permit the build-up of dust and other adverse particlates upon the motor windings. Moreover, the cooling produced by this method is very minimal because relatively little, if any, air ultimately finds its way through the motor.
More recently some manufacturers of blower fans for commercial furnaces have mounted the fan motor partially within the fan blade assembly such that air is rapidly drawn from one end of the motor through openings in the motor housing. Not only does this technique accelerate the collection of dirt inside the motor housing, but heat removed from the motor is collected by the fan and circulated with the main airflow. Although this may be acceptable in the context of furnaces for heating, leaving the motor housing open and circulating the motor generated heat with the main airflow of a ceiling mounted fan increases motor maintenance requirements, reduces its working lifetime, and perhaps most significant serves to add heat to the mainstream of air generated by the fan.